I came across this problem, and on investigation identified that farstream is basically superseding farsight. I un-merged net-libs/farsight2 and net-libs/telepathy-farsight, and then merged net-libs/farstream and net-libs/telepathy-farstream. I re-emerged the programs which used the older packages, and they reverted to using the later version. I didn't need to mask anything.

I apologise if this sounds slightly vague, but I recently removed Gnome from my system, and none of these files are required are longer, so they have all gone.

same here.
ktp-call-ui requires farstream and papyon and pidgin require farsight2.
farsight2 and farstream are blocking each other.

any news?

Actually, from what I can see in the latest ebuilds, ktp-call-ui requires farstream, papyon requires farsight2, but pidgin will work with which ever is installed.

I suggest you push the developers upstream to consider moving to farstream, or better still allowing either to be used, as pidgin does. Seeing which packages you are struggling with explains why I didn't have a problem in the end - clearly whatever wanted access to these libraries was configured to use which ever was available.

I manually deleted all packages depending farsight2 and all packages depending on them, and go on until the dependencies were cleaned. Then re-emerge the world and it succeeded. I don't know why depclean would not remove these packages...

Depclean will never remove packages that are in @system, @world or (possibly deep) dependencies of those. If you want to remove a subtree, you need to obsolete it first by removing the packages that require it (check with equery depends atom). Only when no more packages require the subtree, depclean will unmerge it. Note that if a dependency can be satisfied by different packages, like "farsight | farstream", a package that could be safely removed might still be considered "needed" by depclean, because it does not know which of the two packages satisfied the dependency at compile time.

Some packages still depend on farsight, even though they would work with farstream as well. This is kind of a bug and should be reported to the package maintainer when needed. Papyon seems to need a bit of patching for the migration, haven't tested that yet.